The present invention relates to an apparatus for the production of tubing, rods and the like from crystalline quartz or other glass like materials. Particularly, this invention relates to an apparatus for use in the production of elongated quartz members from a silica melt. The present invention is particularly directed to the manufacture of fused silica tubes for use in the manufacture of optical fibers.
Various types of elongated members have been formed continuously by melting of quartz crystal or sand in an electrically heated furnace whereby the desired shape is drawn from the furnace through a suitable orifice or die in the bottom of the furnace as the raw material is melted. One apparatus for continuous production of fused quartz tubing, for example, is a tungsten-lined molybdenum crucible supported vertically and having a suitable orifice or die in the bottom to draw cane, rods, or tubing. The crucible is surrounded by an arrangement of tungsten heating elements or rods which heat the crucible. The crucible, together with its heating unit, is encased in a refractory chamber supported by a water-cooled metal jacket. The crucible is heated in a reducing atmosphere of nitrogen and hydrogen.
An alternative apparatus provides fused quartz tubing by feeding natural quartz crystal into a refractory metal crucible heated by electrical resistance under a particular gas atmosphere to reduce the bubble content. The bubbles formed by gas entrapment between crystals and the molten viscous mass of fused quartz do not readily escape from the molten glass and, hence, remain as bubbles or lines in the product drawn from the fused quartz melt. By substituting a melting atmosphere gas which readily diffuses through the molten material (such as pure helium, pure hydrogen or mixtures of these gases) the gas pressure in the bubbles is reduced and thereby the bubble size is reduced. This process uses a mixture of 80% helium and 20% hydrogen by volume.
In a further alternative method, a product is obtained by continuously feeding a raw material of essentially pure silicon dioxide in particulate form into the top section of an induction-heated crucible, fusing the raw material continuously in an upper-induction heat zone of the crucible in an atmosphere of hydrogen and helium while maintaining a fusion temperature not below approximately 2050xc2x0 C. The fused material in the lower zone of the crucible is heated by separate induction heating means to produce independent regulation of the temperature in the fused material. The fused material is continuously drawn from the lower zone of the crucible through forming means in the presence of an atmosphere of hydrogen containing a non-oxidizing carrier gas.
Unfortunately, most of the refractory metal and non-metal materials used in the crucibles of the above-described apparatus are undesirable impurities if present in the drawn silica article. Such refractory material contamination causes discoloration and occlusions in the silica glass. Furthermore, the presence of refractory material particles can degrade the strength of the resultant silica article. Moreover, the particles become a flaw in the drawn article that can cause the strand to break.
Accordingly, there is a need in the art to reduce contamination of fused glass occurring from the refractory materials used in constructing the furnace. This need has increased recently as semiconductor and fiber optics manufacturing processes, a primary use for the glass products obtained from the subject process, have required higher levels of purity and performance.
Unfortunately, because the furnace is typically constructed of refractory materials, the manufacturing plant is usually contaminated therewith. Accordingly, even a furnace having melting and drawing zones insulated from refractory materials cannot fully prevent contamination. It would therefore be desirable to have available an apparatus which facilitates removing and/or reducing the effect of refractory materials contamination on the resultant silica article.
In an exemplary embodiment of the invention, a furnace for melting of the silica and subsequent drawing into a desired shape is comprised of a body having an outer surface constructed of a refractory metal and including a substantially gas impermeable inner lining extending from at least slightly below a melt line throughout the melt zone of the furnace. The lining is comprised of a non-reactive barrier material. The lining is preferably formed of rhenium, osmium, iridium, platinum or mixtures thereof. Preferably, the furnace will include an inlet tube for introduction of a gas to the melt zone, the outer circumference of the inlet tube forming a gas-tight seal with the lining. In addition, a silica raw material feeder tube, or a protection sheath circumferentially disposed around the tube is provided, the outer circumference of the feeder tube forming a gas tight seal with the lining.
The present crucible construction provides a number of advantages. Particularly, furnaces constructed with a substantially gas-tight rhenium, iridium, platinum and/or osmium lined melting zone can produce products with much lower levels of refractory metal in the solution. For example, the metal dissolved in the silica can be reduced to below about 1 ppb and preferably below the current level of detection via NAA. This reduced amount of refractory metal contamination in the silica melt improves the chemical composition of the silica glass allowing for a decrease in discoloration and surface haze. Furthermore, utilization of a furnace equipped with a crucible including the non-reactive lining in the melt zone allows operation at optimum temperature ranges and in the presence of a desired atmosphere to reduce or eliminate contaminants.
It should be noted that the terms xe2x80x9cquartzxe2x80x9d and xe2x80x9csilicaxe2x80x9d are used interchangeably throughout this application, both being directed generally to the compound SiO2. Nonetheless, the present invention encompasses the use of any raw material introduced to the melting furnace, including but not limited to natural silica/quartz and synthetic silica.